1. Field of the Invention
The present invention relates to an optical pickup apparatus that carries out actions of reading out a signal recorded on an optical disc and of recording a signal on the optical disc by a laser beam.
2. Description of the Related Art
An optical disc apparatus is in popular use, which is able to carry out a signal reproducing action and a signal recording action by causing a laser beam emitted from an optical pickup apparatus to enter onto a signal recording layer of an optical disc.
While an optical disc apparatus using an optical disc such as a CD and a DVD is now in wide use, an optical disc apparatus using an optical disc with an improved recording density, i.e., a Blu-ray standard optical disc has been produced on a commercial basis in recent years.
A laser beam of a short wavelength, for example a blue-violet light having a wavelength of 405 nm, is used as a laser beam for a reading out action of a signal recorded on the Blu-ray standard optical disc.
The thickness of a protective layer disposed on the upper surface of a signal recording layer of the Blu-ray standard optical disc is provided at 0.1 mm, and the numerical aperture of an objective lens used for an action of reading out a signal from the signal recording layer is provided at 0.85.
The numerical aperture of the objective lens used for an action of reading out a signal recorded on the Blu-ray standard optical disc is determined to be 0.85 as described above, which is relatively large. As a logical consequence, an incident angle of a laser beam against the objective lens becomes large. This large incident angle against the objective lens results in an increase in the quantity of reflection of the laser beam on the outer region of the objective lens, thus leading to a decrease in the quantity of transmitted light on the outer region.
A decrease in the quantity of transmitted light on the outer region of the objective lens leads to a drop in a signal-to-noise ratio in a signal readout action, which poses a problem that the signal readout action cannot be carried out accurately. As a method for solving such a problem there is provided a method of forming an anti-reflection coating on an incident surface of the objective lens (see Japanese Patent Application Laid-Open Publication Nos. 10-160906 and 2008-282507).
The optical pickup apparatus that carries out an action of reading out a signal recorded on the Blu-ray standard optical disc is configured to use a laser diode that emits a blue-violet laser beam having the wavelength of 405 nm and the objective lens having the numerical aperture of 0.85, as described above. When a laser beam transmittance is not high at a position at which the numerical aperture of the objective lens is 0.85, the rim intensity of a laser spot generated by a condensing action of the objective lens decreases.
A decrease in the rim intensity of the laser spot causes the spot diameter, i.e., spot size to increase, which poses a problem of a drop in resolution for recognizing pits formed on the optical disc.
To solve such problems, the anti-reflection coating is formed on the incident surface of the objective lens to suppress a decrease in the quantity of transmission of a laser beam on the outer region. The objective lens is designed so that the transmittance in a range of use of the objective lens, that is, the transmittance at the numerical aperture 0.85 determined in correspondence to the Blu-ray standard optical disc becomes the maximum, and that the range of use up to the numerical aperture 0.85 acts as a lens surface.
An objective lens or a lens holder, however, has a manufacturing tolerance. This manufacturing tolerance makes it difficult to maximize the transmittance at the numerical aperture of 0.85 and makes it impossible to manufacture the objective lens or lens holder so that the range up to the numerical aperture 0.85 acts as the lens surface. Hence a problem arises such that a laser spot applicable to the action of reading out a signal recorded on the optical disc cannot be generated.